Boiler thermostats - how exactly do they work?

[happytotryit]

Boiler thermostats have been much discussed but I haven't found the precise answer to my query. I know the stat determines the heat of the water flowing out of the boiler. But how?

My understanding is that the boiler burners always operate at the same pressure and that different water temperatures are achieved by the boiler thermostat switching the burners off when the desired (boiler) temperature has been reached - irrespective of whether the room thermostat is still calling for heat. So at a low setting on the boiler dial, the room temperature may never be reached, whereas with a high setting, the room thermostat may switch the burners off before the boiler thermostat does.

So is this right or wrong? Wrong, according to Ideal Technical Helpline. They told me that the boiler is only triggered on and off by the room stat (and this will happen sooner if I turn the boiler stat up). If this is true, what is the function of the switch within the boiler thermostat and when would this be activated?

The thermostat on my Ideal Minimiser SE60 condensing boiler seems to me to have stopped switching off so often - even at a setting below 1 it can be over half an hour before the boiler switches off, and as long again before it switches back on. Nevertheless it is the boiler stat that is switching the boiler off - the radiators are very hot but the room temp has not yet reached the room stat setting (turning the room stat down does turn the boiler off). So that seems to contradict Ideal's explanation.

I have tried running the boiler at maximum (number 6). Even after an hour the boiler had not switched off once. So I turned the boiler stat dial back and it clicked off at just below number 2. But turning the it back up, it did not click on again until I reached maximum.

Any light shed on this gratefully received!

Thanks.

 

[D_Hailsham]

I know the stat determines the heat of the water flowing out of the boiler. But how?

There is a sensor in the flow pipe which measures the water temperature. This is connected, in your case by a capillary tube, to a variable resistor (temperature control) and switch which opens at the set temperature and turns the gas supply off. When the water temperature has dropped about 10C the switch will close and the boiler restarts.

My understanding is that the boiler burners always operate at the same pressure and that different water temperatures are achieved by the boiler thermostat switching the burners off when the desired (boiler) temperature has been reached - irrespective of whether the room thermostat is still calling for heat.

That's true of your boiler. but modern modulating boilers will turn the gas pressure down, so the boiler produces less heat, to maintain a constant flow temperature. If it reaches the minimum output and the water temperature continues to rise, the boiler will go out until the water temperature has dropped a little. It then starts up again.

So at a low setting on the boiler dial, the room temperature may never be reached, whereas with a high setting, the room thermostat may switch the burners off before the boiler thermostat does.

Correct. At lower temperatures the rads may not be producing enough heat to raise the room temperature enough, so the room stat is continually calling for heat.

Wrong, according to Ideal Technical Helpline. They told me that the boiler is only triggered on and off by the room stat (and this will happen sooner if I turn the boiler stat up).

Ideal are assuming that the boiler will give of enough heat, whatever the water temperature. This may be true in the spring, when it's 10C or more outside and not much heat is required, but not when it's freezing outside and the full output of the boiler is required so the rads are producing their stated output. (Simply put, rad output varies with water temperature.)

I suspect that your boiler stat is faulty. They are still available.

 

[happytotryit]

I suspect that your boiler stat is faulty.

Thanks for your comprehensive reply. You've restored my faith in my own memory. Clearly the Ideal Helpline response to me was not thought through sufficiently.

So am I correct in thinking (and recalling) that over an hour you would expect the boiler stat to turn the boiler off and on a few times (if the room stat didn't switch it off first)?

I've looked at the servicing manual and replacing the thermostat looks like one of the few things one could tackle oneself. Maybe I'll give it a whirl. Any likely pitfalls...?

 

[Gas2Air]

The pitfall being it's a Minimiser. Yes the blr stat will cut in and out due to water temperature. Regardless of room stat. If the room stat is demanding 20c then the blr and radiators are trying to hit that.

Change the boiler stat. But I suspect a scaled hex.

 

[ianmcd]

as advised the heat exchanger on theses MUST be thoroughly cleaned every 2 years they are an absolute pain to do and you wouldnt be able to do it yourself

 

[happytotryit]

the heat exchanger on these MUST be thoroughly cleaned every 2 years they are an absolute pain to do and you wouldnt be able to do it yourself

Nor would I wish to! Point taken. Somewhat different to the old days of wire-brushing the rust off the fins of a cast iron heat exchanger...

Change the boiler stat. But I suspect a scaled hex.

Sorry, are you saying a scaled heat exchanger may be the problem, or just an additional one (possibly exacerbated by a faulty thermostat)? As I said, to manually switch the burners off and on I turned the boiler stat dial back from maximum (6) and it clicked off at just below number 2. But turning it back up, it did not click on again until it reached maximum. Those clicks should be closer together shouldn't they?

Thanks.